1. Field of the Invention
The invention relates to the manufacture of a high optical quality transparent plastic sheet which can be used alone or in association with other materials, particularly in laminated glazings.
2. Discussion of the Background
Plastic sheets capable of being used in laminated glazings of the type mentioned above have already been proposed. For example, in British Patent No. 1576394, a sheet is described which has two layers (1) a layer of a thermoplastic material which, in the laminated glazing application comprising a single sheet of glass, is an interpolated layer with energy-absorbing properties, and (2) a layer of heat-hardenable material with anti-lacerating and auto-cicatrizing properties, such that it is resistant to scratching and abrasion.
The interpolated layer with energy-absorbing properties is a thermoplastic polyurethane obtained from at least one aliphatic diisocyanate and at least one polyesterdiol or polyetherdiol, with a ratio of NCO groups to OH groups of between 0.8 an 0.9. Glazing using such a two-layer sheet retains good optical properties and adhesion between the elements, but the bio-mechanical properties of the glazing and, in particular its shock-resistance, are not completely satisfactory. On the other hand, the two-layer plastic sheet can be rolled and easily handled without deterioration of its optical quality.
European Patent No. 0 133 090 further discloses a high optical quality transparent sheet which can be used alone or in association with other materials particularly in the manufacture of the laminated glazings described above. This sheet comprises a layer formed in a continuous process by reactive pouring onto a flat horizontal support, from which it is detachable, of a reaction mixture of an isocyanate component and a component with active hydrogens, in particular a polyol component. The isocyanate component comprises at least one aliphatic or cycloaliphatic diisocyanate or a diisocyanate prepolymer and this component has a viscosity of below approximately 5000 centipoises at +40.degree. C. The polyol component comprises at least one difunctional long polyol with a molecular weight of between 500 and 4000 and at least one short diol as a chain extending agent. Reactive pouring signifies pouring in the form of a layer or a film of a liquid mixture of the components in the monomeric or prepolymeric state, followed by the polymerization of said mixture using heat.
Safety glazing using such a layer has good biomechanical properties under variable conditions of temperature and humidity. However, when the sheet is rolled to form reels, using an interpolated film which is generally polyethylene, and then the sheet is subsequently unrolled before use, marks or impressions left by the interpolated film are observed on the polyurethane layer with energy-absorbing properties (more simply designated as the "EA layer").
These marks cannot subsequently be removed completely, even when the plastic sheet is subjected to a heat cycle of several minutes at a temperature above 100.degree. C., for example 10 minutes at 120.degree. C.
Marking of the plastic sheet occurs regardless of the degree of polymerization of the EA layer. The marking is, however more accentuated when the degree of polymerization is lower. To determine the degree of polymerization of the EA layer, a simple method consists of measuring the ratio between the height of the NCO peaks and the height of the CH peaks of the infrared spectrum of the EA layer upon exiting the polymerization area. It is considered that the EA layer is completely polymerized when the NCO peak has disappeared.
In effect, it would seem that the polymerization or the aging of the layer is never total at the end of an assembly line. To obtain complete polymerization, a polymerization heat cycle must be used which is much longer than the cycles normally used in industrial production or higher temperatures must be used.
Thus, the problems which remain unsolved by the prior art are the following. When a two-layer sheet is utilized which retains good optical properties and adhesion, there is unsatisfactory biomechanical properties present such as a decrease in shock resistance.
When a layer is prepared with satisfactory biomechanical properties, there occurs extensive marking as a result of the rolling/unrolling of the sheets.